1. Field of the Invention
This invention relates to a signal encoding apparatus for compressing and encoding input signals, such as voice signals, and, more particularly, to a signal encoding apparatus for orthogonally converting and encoding input signals.
2. Description of the Prior Art
In orthogonal conversion processes employed in conversion encoding which is among the signal compression encoding techniques, specific techniques include discrete Fourier transform (DFT), Hadamard transform, Karhunen Leve transform (KLT), discrete cosine transform (DCT) and Legendre transform. These orthogonal transforms convert the sample values into mutual orthogonal axes for removing (or reducing) correlation among the sample values or concentrating the signal energies to certain coefficients and represent one of the compression artifices for data, such as sound or video image.
Among these orthogonal transforms, the above discrete Fourier transform is such a transform in which the signal after the transform is an expression on the frequency axis of the original signal expression on the time axis, so that the control of the S/N ratio on the frequency axis at the time of quantizing the converted signal is facilitated. Thus, in application to voice signals, encoding under utilization of the characteristics or the frequency axis of the human auditory sense is possible so that the S/N ratio may be improved as long as the auditory sense is concerned. In application to video signals, human visual characteristics differ between the low frequency component and the high frequency component so that, after converting the input video signals into these components, encoding suited thereto may be made to realize effective compression.
In such an orthogonal compression system, conversion is on the block-by-block basis, with input signals being divided at predetermined lengths on the time axis or on the frequency axis, with the block length being of a constant value. This block length has been determined in consideration of the statistic properties of the input signals, that is, the properties of typical input signals. For example, in the case of musical signals, the block length is determined on the basis of the above mentioned human auditory properties, whereas, in the case of video signals, the block length is determined on the basis of the human visual properties.
However, in effect, actual input signals, such as voice or image signals, are changed in their properties, such as in levels, so conspicuously with time that the preset block length may not be optimum at a certain time. Therefore, when such input signal is orthogonally converted at the predetermined length of the block and the thus converted signal is decoded, the S/N ratio may be degraded.